1. Field of the Invention
The present invention concerns broad-band differential amplifiers, adapted, for example, to operating up to frequencies of about 20 megahertz.
2. Discussion of the Background
A known type of amplifier is represented in FIG. 1; it comprises a differential stage for the obtention of a desired gain and a voltage follower stage, of unitary gain, for impedance matching with respect to the load that will be connected to the output. Furthermore, the amplifier comprises a common mode feed-back branch through which is reinjected in a certain way in the differential stage a signal representing the amplitude of the signal present on one of the outputs of the differential stage.
More specifically, in the diagram of FIG. 1, the differential stage comprises a common bias branch supplying two differential branches with current; the differential branches each comprise an input MOS-transistor (T1 for the first branch, T2 for the second) in series with a load-MOS-transistor (T3, T4 respectively) having its gate connected to its source if it is a transistor of the depletion type (negative voltage threshold) or its gate connected to its drain if it is a transistor of the enhancement type (positive voltage threshold); in the present description, T3 and T4 are depletion transistors; inputs E1 and E2 of the differential stage are gates of transistors T1 and T2; outputs S1 and S2 are respectively the junction point of transistors T1 and T3 of the first branch and the junction point of transistors T2 and T4 of the second branch.
The common bias branch is constituted by a MOS-transistor T5 connected between a first supply terminal A of the circuit and the combined sources of the input transistors T1 and T2.
The differential branches are connected by the drains to load-transistors T3 and T4 to a second supply terminal B.
The impedance matching follower stage comprises two MOS-transistors T6 and T7 in series between the terminals A and B, transistor T7 connected to terminal A, receiving on its gate the first output S1 of the differential stage. Output S of the amplifier is taken at the junction point of the drain of transistor T7 and of the source of transistor T6.
The common mode feed-back branch, which acts to stabilize the output levels on terminals S1 and S2 by modifying the current in the common bias branch, comprises two transistors T8 and T9 assembled as a follower stage between the second output S2 of the differential stage and the control gate of transistor T5 of the common branch; transistor T8 has its drain connected to terminal B, its gate to point S2, its source to the drain of transistor T9 and to the gate of transistor T5; transistor T9 has its source connected to terminal A and its gate supplied by a bias voltage that can be the same (Vp) as that of transistor T7.
In other lay-outs of amplifiers it is known that it is also possible to provide a double common mode feed-back by positioning another transistor in parallel on transistor T5, this other transistor having its gate controlled by output S1 through a voltage follower.
Furthermore, a basic problem in broad-band amplifiers is that of preventing oscillation at high frequencies when they are looped in unitary gain. For example, an amplifier having a gain higher than one in a bandwidth of about 20 MHz could cause a risk of oscillation at 80 MHz if its frequency response curve is such that at 80 MHz it has a gain higher than or equal to 1 while having a phase rotation of 180.degree. at this frequency.
In order to prevent this type of oscillation, it is necessary to artificially reduce the principal cut-off frequency of the amplifier; by way of example, FIG. 1 represents the introduction of a capacitor C1 (in dashed lines) which it has been tried to position between output S1 (that which attacks the follower stage) and the second supply terminal B in order to reduce the oscillation risk.
But another difficulty arises; by introducing this capacitor there is an overall reduction of the bandwidth of the amplifier and therefore the desired specifications are not satisfied, especially with respect to the rise time of the amplifier. Thus, by wishing to suppress the oscillation risk at 80 MHz for an amplifier having a bandwidth extending up to 20 MHz, it is necessary to reduce the bandwidth to below 20 MHz, which it was not desired to do.
This problem is very general for broad-band amplifiers and cannot be overcome altogether satisfactorily with the risks of oscillation appearing being increased as both the bandwidth and the gain are raised.
Only compromises allow the set specifications to be achieved.
The present invention proposes a novel solution to overcome the risk of oscillation by maintaining the product (gain.times.bandwidth) as broad as possible (this product being a characteristic value for the performances of the amplifier).
According to the invention, in an amplifier such as that which has been described herein-above with reference to FIG. 1, the following are provided: on the one hand, a second common mode feed-back branch, similar to the first, between the first output S1 of the differential stage and the gate of a transistor positioned in parallel with transistor T5 of the common bias branch, and on the other hand, a capacitor between the second output S2 and the second supply terminal B, this capacitor being such that the equivalent capacitance between the second output S2 and the second supply terminal B is distinctly higher than the equivalent capacitance between the first output S1 and the second supply terminal B.
The reduction of the risk of oscillation is particularly important if the conductance of the load-transistors T3, T4 is given a value clearly lower than the conductance of the input transistors T1, T2.
The range of size of the equivalent capacitance between S2 and B can be about 3 to 6 times the equivalent capacitance between S1 and B.